Experimental Study On The Seismic Performance Of BRB Retrofitted RC Frames

With the development of society and earthquake engineering,energy dissipation design and energy dissipaters have been widely applied to both the design of new buildings and rehabilitation of existing structures.As a type of energy dissipater with simple configuration and stable performance,Buckling-Restrained-Braces,known as BRBs,have been widely used in steel and RC structures.However,with the absence of specific design provisions of BRBs in the European seismic code EuroCode8,the application of BRBs in related area is obviously hampered.In view of this absence,M.Bosco et al proposed a new design method for BRBs used in steel structures under the framework of EC8,and later extended it into RC structure rehabilitation.The design method had only been tested by numerical simulation,thus its effectiveness still needs to be tested by specimen tests,which is the main work of this dissertation.There are two prototype structures with 6 stories,both originated from Italy.Rehabilitation design was carried out on the two prototype structures using the method above,with only diagonal configuration for Prototype I and both diagonal and double-K configuration for Prototype II.To verify the performance of the rehabilitation,both numerical analysis and hybrid test were conducted on the original and retrofitted structures.The main content of this dissertation are as following:Firstly,numerical simulation of the original and retrofitted structures were conducted.Ten ground motions were selected and scaled to the PGA of 0.35g and 0.45g,corresponding to Significant Damadge(SD)and Near Collapse(NC)limit state,respectively.Nonlinear time history analyses were conducted on the original and retrofitted structures of both Prototype I and Prototype II.Results show that the inter story drift ratio(IDR)of retrofitted structures are all reduced with more uniform distribution along the height.The max reduction is over 70%,meaning that the method is effective.Then,an artificial ground motion was selected for the hybrid test.The equation of motion was simplified as a 6DOF system.The tested substructure was selected as the side span at the first story and the rest of the frame was numerically modeled in OpenSEES.The two substructures were coordinated in a staggered pattern to maintain the equilibrium and compatibility on the boundaries between the two substructures.During the test,the ground motion was scaled to the PGA of 0.1g,0.2g,0.35g,0.45g and 0.62g to examine the seismic performance of the structures.Results of the test show that the weak story of Prototype I is at the 4th story because of the irregularity of stiffness introduced by the change of cross section of columns,while for Prototype II,the weak story is at the first story.For retrofitted frames,the IDR of both prototypes decrease significantly and their distribution get more uniform.Besides,the retrofitted structures can resist the intensity larger than original structures and are eligible for the provisions of EC8.It is also indicated that the two configurations of BRBs on Prototype ?,diagonal and double-K,both work well and are generally equivalent.During the test,damages on the anchorage were observed.For Prototype?,the anchorage at the top gusset started to slip at the PGA of 0.45g,and all the bolts were pulled out and sheared off at the PGA of 0.62g,with the cover concrete of the column crushed.This failure of anchorage led to the failure of the connection between the BRB and RC elements,making the BRB a Compresion Only element.For Prototype ?,the anchorage worked well during all the pseudo dynamic tests,ensuring the collaborative work of BRBs and the RC part.The anchorage failed during the cyclic quasi-static test post the hybrid tests.The bolts at lower beam gusset were sheared off and the lower two BRBs were out of work.The bolts at this place were only sheared off without pulling out,thus the concrete of the beam was not seriously damaged and only the grouting cracked.With the numerical analysis and hybrid test on the two prototypes with two configurations of BRBs,the performance of the rehabilitation and the effectiveness of the design method are both confirmed.The research also found that the anchorage need to be strengthened to ensure the effectiveness and reliability of the retrofitted system during very rare earthquakes.